The building code required a voice evacuation system. The system was installed, tested for SPL (passed), and certified. Nobody tested it for intelligibility. During the first real alarm, the pre-recorded message "Attention: a fire emergency has been reported. Please proceed to the nearest exit" was heard by occupants as "AAATENNNN... FIRRR MERRRRGENNN... PLEEEEEZ PROOOCEEED... NEEEERRRESSS EGGGSIT." People froze.

Some occupants reported hearing the word "fire" and evacuated. Others heard "proceed" and assumed it was a drill. A group on the 7th floor heard "nearest exit" as "nearest desk" and sheltered in place. The evacuation took 14 minutes instead of the designed 6 minutes because nobody understood the instructions.

Post-incident measurement revealed an STI of 0.22 in the corridors and 0.18 in the stairwells. IEC 60268-16 and EN 54-16 require a minimum STI of 0.50 for voice alarm systems. The system was 0.28 points below the legal minimum — technically non-compliant but nobody had ever measured it. The speakers were spaced for SPL coverage, not intelligibility coverage. Stairwells, with their hard parallel surfaces, were acoustic nightmares.

Voice evacuation systems save lives only if the message can be understood. SPL alone is meaningless if the words are garbled.

The Moral: SPL compliance is not intelligibility compliance. Every voice alarm system must be STI-verified. Use SonaVyx STI to measure at every listening position — lives depend on it.